Process for preparing a synthetic intermediate for preparation of branched nucleosides

ABSTRACT

A process is provided for the preparation of a key intermediate in the preparation of 2′-branched nucleoside compounds. The process includes contacting a protected precursor 3,4-O-isopropylidene-2-C-substituted- D -arabinono-1,5-lactone with a fluorinating agent under anhydrous conditions and converting the precursor into a protected 2-deoxy-2-halo-2-C-disubstituted ribono-1,5-lactone and optionally into a 2-deoxy-2-halo-2-C-disubstituted ribono-1,4-lactone.

CROSS REFERENCE

This application claims priority to U.S. Provisional Application No. 60/753,507 filed Dec. 23, 2005, the disclosure of which is incorporated by reference.

FIELD OF THE INVENTION

This application relates to methods for the preparation of a synthetic intermediate in a process for preparing 2′-C-alkyl-2′-halo-nucleoside analogues, which are important as anti-viral, anti-cancer, and antibacterial agents.

BACKGROUND OF THE INVENTION

A process for preparing a halogen-substituted ribonolactone intermediate that is useful in the synthesis of a 2′-C-methyl-2′-halo nucleoside analogue presents ongoing challenges, particularly where the halogen atom is fluorine.

A key intermediate in the preparation of sugar analogues used in the synthesis of nucleosides and vitamins is 2-C-methyl-D-ribono-lactone. As early as 1880, Scheibler described a process for preparing the lactone (John Sowden, “The Saccharinic Acids” in Adv. Carbohydrate Chem. 12:43-46 (1957), citing C. Scheibler, Berichte 13:2212 (1880)). Unfortunately, product yields were only approximately 10% (Id.). At about the same time, H. Kiliani synthesized 2-methyl-D-ribonolactone by treating D-fructose with calcium hydroxide (H. Kiliani, Berichte, 15:2953 (1882), as cited in F. J. Lopez-Herrera et al., J. Carbohydrate Chemistry, 13(5):767-775 (1994)). However, the process required months to run to completion and product yield was also only approximately 10% (Id. at 768). Kiliani's process, however, enabled him to establish the positions of important functional groups on the compound (John Sowden, “The Saccharinic Acids” in Adv. Carbohydrate Chem. 12:43-46 (1957), citing H. Kiliani, Ann., 213:361 (1883)).

In the early 1960s, Whistler and BeMiller attempted to improve upon Kiliani's synthesis (Roy L. Whistler and J. N. BeMiller, “α-D-Glucosaccharino-1,4-lactone” in Methods in Carbohydrate Chemistry, 2:484-485 (1963)). Whistler and BeMiller added boiling water and calcium hydroxide to D-fructose, flushed the system with nitrogen gas, and repeated the same process. The mixture then was maintained for 6-8 weeks, after which it was treated with CO₂ and oxalic acid dihydrate, and filtered under pressure. The residue washed repeatedly to a syrup-like consistency, and filtrates combined; solvent evaporated under reduced pressure and the resultant product allowed to crystallize under refrigeration. The final product yield was still only about 10% (Id. at 485) and the process took two months to complete.

BE 731271 and GB 1189973, assigned to Deutsche Akademie der Wissenchaften, disclosed a process for preparing 3′-fluoronucleosides by reacting a nucleoside with a fluorinating agent such as HF in an organic solvent like THF at temperatures ranging from 130-160° C.

In an attempt to improve product yields, Lopez-Aparicio et al. reported the synthesis of 2-C-methyl-D-ribono-1,4-lactone from 2,3-O-isopropylidene-D-glyceraldehyde as an alternative to the Kiliani synthesis (Lopez-Aparicio et al., Carbohydrate Res., 129:99 (1984), as cited in F. J. Lopez-Herrera et al., J. Carbohydrate Chemistry, 13(5):767-775 (1994) at 768-769). The process of Lopez-Aparicio included condensing 2,3-O-isopropylidene-D-glyceraldehyde with (1-methoxy-carbonyl-ethylidene)triphenylphosphorane to produce methyl E-(S-4,5-dihydroxy-4,5-O-isopropylidene-2-methyl-2-pentenoate; hydrolyzing (in HCl) and photochemically isomerizing the pentenoate; lactonizing the pentenoate product to produce a butenolide; tritylating the butenolide at C-5 by reaction with trityl-chloride and pyridine, followed by cis-hydroxylation with potassium permanganate and methylene chloride in the presence of a crown ether. Final removal of the trityl(triphenylmethyl) group was achieved by reaction with TFA (trifluoroacetic acid) (Id. at 768). Lopez-Aparicio et al. reported product yields of ribonolactone at about 80%, but others were not able to reproduce this figure based on the gram mass amounts of materials provided in the experimental section of their publication. Instead, calculations indicated a percent yield of about 36% ribonolactone. In addition, the process of Lopez-Aparicio et al. was far more complex than the Kiliani synthesis, required the use of toxic reagents such as potassium permanganate and specialized equipment for irradiation to attain photochemical isomerization, and had a minimum of 60 hours reaction time (Id. at 768, 770-772).

None of the foregoing approaches addressed the problem of preparing 2′-C-branched or 2′-disubstituted ribonucleoside analogues.

In 1989, the Asahi Glass Company Ltd. reported the synthesis of fluoronucleosides that had antiviral and antitumor effects (JP 02270864 and JP 01100190). These nucleosides were prepared by treating a 9-(alpha-fluoro-4-beta-hydroxy-1-beta-cyclopentyl)pyrimidine derivative with trifluoromethanesulphonyl chloride, p-toluenesulphonyl chloride, methanesulphonyl chloride or imidazolylsulphonyl chloride in the presence of a base, followed by reduction (JP 02270864). In a second synthetic method, 2′,3′-deoxy-2′,3′-didehydro-2′-fluoronucleosides were obtained by the dehydrogenation of a 2′-deoxy-2′-fluororibofuranosyl derivative, or by dehydrogenation of a 2′,3′-dideoxy-2′-fluoro-3′-halo-ribonucleoside derivative (JP 01100190).

In 1990, Bobek et al. disclosed the synthesis of antiviral, antitumor, and antimicrobial arabinopyranosyl nucleoside derivatives that had a fluorine atom at the 2′-position of the pyranose ring (U.S. Pat. No. 4,918,056). These compounds were prepared by the condensation of a pyrimidine, purine, or 1,3-oxazine nucleobase with an hydroxyl group-blocked, acylated 2-deoxy-2,2-difluoro-D-arabinopyranoside and/or an acylated 2-deoxy-2-bromo-2-fluoro-D-arabinopyranoside (Id.).

In 1997 Harry-O'Kuru et al. described a synthetic route for preparing 2′-C-branched ribonucleosides (Harry-O'Kuru et al., J. Org. Chem., 62:1754-9 (1997)). Commercially available 1,3,5-tri-O-benzoyl-α-D-ribofuranose was used as the starting material, which was prepared from D-ribose or D-arabinose (D-arabinopyranose). The 1,3,5-tri-O-benzoyl-α-D-ribofuranose was oxidized at the free 2-OH with Dess-Martin periodinane reagent, and produced 1,3,5-tri-O-benzoyl-2-keto-ribofuranose as well as its corresponding hydrate. The desired product and hydrate were stirred with excess MgSO₄ and permitted to stand overnight. The mixture was then filtered and concentrated in order to produce a substantially pure ketone product. The resultant 2-ketosugar was treated with MeMgBr/TiCl₄ (or alternatively with MeTiCl₃, CH₂═CHMgBr/CeCl₃, or TMSC≡CLi/CeCl₃), which produced an anomeric mixture of the desired 1,3,5-tri-O-benzoyl-2-substituted alkyl-, alkenyl- or alkynyl-ribofuranoside and its transesterified isomers, α- and β-2,3,5-tri-O-benzoyl-2-substituted alkyl, alkenyl or alkynyl ribofuranoside in a nearly 5:3 ratio of desired product to isomeric forms (Id. at 1755). The 2-alkylated ribofuranosides then were converted to a single, desired product, 1,2,3,5-tetrabenzoyl-2-alkylribofuranoside, by treatment with benzoyl chloride, DMAP and triethylamine in a reported approximately 70% yield with a β/α ratio of 4:1 (Id.).

In 1998, Chambers et al. reported the synthesis of 2′,3′-dideoxy-3′-fluorouridine compounds by reaction of a corresponding anhydronucleoside with hydrogen fluoride in the presence of an organo-iron compound and in an organic solvent (U.S. Pat. No. 5,717,086).

Recent reports of syntheses of 2′ and/or 3′ halonucleosides have been disclosed by Pharmasset, Inc., The University of Georgia Research Foundation, Inc., and Emory University.

WO 05/003147 (also US 2005/0009737) to Pharmasset, Inc., described the synthesis of 2′-C-methyl-2′-fluoro nucleoside analogues by one of two general synthetic routes: alkylating an appropriately modified carbohydrate compound, fluorinating it, and then coupling it to a desired nucleobase, or glycosylating a desired nucleobase to form a nucleoside, then alkylating the nucleoside, and finally fluorinating the preformed nucleoside. Pharmasset's first approach utilized a modified carbohydrate that was an hydroxyl group-protected lactone, which was alkylated with a reagent such as methyl lithium in an anhydrous solvent like THF, and then was reacted with a commercially available fluorinating agent like DAST or Deoxofluor, followed by a deprotection step. The reaction proceeded with inversion at the 2′-position such that the fluorine atom was in the “down” or ribo configuration. Pharmasset's second synthetic route comprised the modification of a commercially available nucleoside whose hydroxyl groups were protected by protective groups known in the art. The nucleoside was oxidized at the 2′-position to provide a 2′-ketone, and the 2′-ketone was reacted with an alkylating agent such as methyl lithium in THF at about 0° C. to afford a 2′(S) or 2′-methyl “down”, 2′-hydroxyl “up” configured nucleoside tertiary alcohol. A fluorine atom then was introduced by reacting the nucleoside with a commercially available fluorinating reagent such as DAST in an anhydrous, aprotic solvent like toluene with inversion at the 2′-position to afford a 2′-C-methyl “up”, 2′-fluoro “down” configuration of the nucleoside. However, by either synthetic route, Pharmasset's isolation and purification methods were impractical/inefficient and product yield was very low in all examples provided.

PCT Publication No. WO 2006/031725 to Pharmasset, Inc. describes the synthesis of 2′-C-substituted-2′-deoxy-2′-halo nucleosides via the nucleophilic ring opening of a 5-membered ring cyclic sulfate intermediate derived from 4,5-di-O-protected-2-methyl-2,3-dihydroxy-pentanoic acid with fluoride to produce a 2-methyl-2-fluoro 4,5-di-O-protected fluorinated acyclic sulfate ester compound. The fluorinated sulfate ester is treated with acid to deprotect the 4,5-hydroxyl groups and cyclized to 2′-fluoro-2′-C-methyl-γ-ribonolactone. The ribonolactone is then converted to the 2′-C-methyl-2′-deoxy-2′-halo nucleosides by reduction of the lactone and coupling with an appropriate base.

WO 2006/012440 to Pharmasset, Inc. describes the synthesis of 2′-C-substituted-2′-deoxy-2′-halo nucleosides via a 2′-fluoro-2′-C-substituted-γ-ribonolactone intermediate. The 2′-fluoro-2′-C-substituted-γ-ribonolactone is formed by cyclization of a 2-fluoro-4,5-di-O-protected-2,3-dihydroxy-pentanoic acid ethyl ester intermediate upon treatment with acid. The fluorination reaction is achieved by treating 4,5-di-O-protected-2-hydroxy-3-O-protected-pentanoic acid ethyl ester with DAST.

Otto et al. reported the synthesis and use of beta-2′ or beta-3′-halonucleosides for the treatment of HIV, hepatitis B, and undesired cellular proliferation (U.S. Pat. No. 6,949,522). The syntheses disclosed produced 2′,3′-dideoxy, 3′,3′-dihalo nucleosides from glyceraldehyde or a sugar ring starting material; 2′,3′-dideoxy, 3′-halo nucleosides from a lactol starting material; and 2′,3′-dideoxy-2′-halo nucleosides from glyceraldehyde as a starting material that proceeds via a lactone intermediate that is selectively reduced to afford a 2′,3′-hydro product.

Clark et al. disclosed the synthesis and antiviral activity of 2′-deoxy-2′-fluoro-2′-C-methylcytidine as an inhibitor of hepatitis C virus (Clark et al., J. Med. Chem. 2005, 48:5504-5508). Synthesis of the product compound proceeded through N⁴-benzoyl-1-(2-methyl-3,5-di-O-benzoyl-β-D-arabinofuranosyl)cytosine as a key intermediate, which was oxidized to the corresponding 2′-ketone derivative by reaction with trifluoroacetic anhydride in DMSO under Swern oxidation conditions. The 2′-ketone derivative was reacted with methyllithium at −78° C. in diethyl ether to afford protected 1-[2-C-methyl-3,5-O-(tetraisopropyldisiloxane-1,3-diyl)1-β-D-arabinofuranosyl]cytosine, and the 3′,5′-silyl protecting group was removed by reaction in TBAF/acetic acid. Clark et al. warn against the use of DAST for fluorination of tertiary alcohol groups because the reaction is substrate specific and stereochemically unpredictable.

Shi et al. reported the syntheses and antiviral activities of a series of D- and L-2′-deoxy-2′-fluororibonucleosides in a hepatitis C replicon system (Shi et al., Bioorganic & Medicinal Chemistry (2005), 13:1641-1652). The halo-substituted nucleosides tested had a single halo substituent at the 2′-position on the nucleoside sugar, and were prepared by direct conversion of D-2,2′-anhydrocytidine to (2′R)-D-2′-deoxy-2′-fluorocytidine by reaction with potassium fluoride and crown ether according to the method of Mengel and Guschlbauer (Angew. Chem. Int. Ed. Engl. 1978, 17:525).

There remains a need for discovering improved synthetic routes and new synthetic intermediates in the preparation of 2′-C-methyl-2′-halo-nucleoside analogue derivatives.

It is an object of the present invention to provide a stereochemically predictable and reliable process for the selective addition of alkyl and halo substituents at the 2′-C-position of a nucleoside analogue.

It is another object of the present invention to provide an efficient process that utilizes a minimum number of steps and a readily available, inexpensive starting material for preparing a key intermediate in the synthesis of a 2′-C-methyl-2′-halo-nucleoside analogue.

It is still another object of the present invention to provide a process that employs non-toxic reagents and provides the key intermediate in good percent product yield.

SUMMARY OF THE INVENTION

Historically, the addition of halogen atoms, particularly fluorine, has presented a challenge for researchers attempting to find a direct and simplified process for adding the atom to a precursor compound to provide a 2-deoxy-2-halo-2-C-substituted-D-ribono-1,4-lactone that then can be used in the synthesis of nucleoside analogue derivatives. In particular, the desired stereochemistry of the halogen and other substituents (typically alkyl) at the 2 position has been difficult to produce because of the competition between substitution with the halogen and elimination of the leaving group at the tertiary carbon to produce a byproduct with an exocyclic double bond. A process that allows nucleophilic substitution with a halogen atom to overtake elimination in this balance has been lacking. It now has been found that a halogen, and particularly a fluorine atom can be efficiently introduced at a tertiary center of an isopropylidene-arabinono-1,5-lactone to afford a 2-C-substituted-2-fluoro-ribonolactone using the process provided herein.

In one embodiment, it was surprisingly found that 2-deoxy-2-fluoro-2-C-substituted lactone compounds can be produced in high yields by nucleophilic displacement on 3,4-O-isopropylidene-2-C-substituted-D-arabinono-1,5-lactone or a halogenated derivative thereof using certain fluorinating reagents under anhydrous conditions.

In one embodiment, the invention provides a process of producing a 2-halo, and particularly 2-fluoro-2-C-substituted-1,5-lactone compound which includes:

a) providing a compound of structure (i) or (ii)

where OR is a suitable leaving group; R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl, including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle; and X is a halogen atom;

b) contacting the compound with a fluorinating agent under anhydrous conditions.

In certain embodiments, the fluorinating agent is tris(dimethylamino)sulfonium difluorotrimethyl silicate (TASF).

In one embodiment, OR is a trifluoromethanesulfonate ester triflate). In another embodiment, OR is a methanesulfonate ester (mesylate). In yet another embodiment, OR is a p-toluenesulfonate ester (tosylate).

In certain embodiment, the reaction contains less than 1%, less than 0.1%, or less than 0.01% water.

In certain embodiments, the process produces a compound of Formula (II)

wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl, including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle, in at least 40% or more yield.

In certain subembodiments, the compound of Formula (II) is produced in at least 50%, at least 55%, at least 60%, at least 70% or at least 80% or more yield.

In one subembodiment, R¹ in the compound of Formula (II) is methyl. In another embodiment, R¹ is ethyl. In another embodiment, R¹ is vinyl. In yet another embodiment, R¹ is —C≡CR², wherein R² is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, CF₃, cyano, aryl, benzyl, or heterocycle. In another embodiment, R¹ cyano. In another embodiment, R¹ is benzyl. In yet another embodiment, R¹ is a heterocycle.

In one embodiment, the process further includes converting a compound of Formula (II) to a 1,4-lactone compound. In one embodiment, this conversion includes contacting the product from step (b) with an acid in a suitable organic solvent. In one embodiment, the acid is an organic acid. In one subembodiment, the acid is trifluoroacetic acid. In another subembodiment, the acid is acetic acid. In another embodiment, the acid is an aryl or alkyl sulfonic acid. In one subembodiment the solvent is 1,4-dioxane. In one embodiment, the 1,4-lactone product can be a 2-deoxy-2-halo-2-C-methyl-D-ribono-1,4-lactone of Formula (B):

wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle.

In certain embodiments, these compounds can be further modified by reduction of the lactone, derivitization of the resulting hydroxyl to a suitable leaving group and substitution with a base (including purine or pyrimidine bases) to provide a 2′-fluoro-2′-branched nucleoside including 2′-deoxy-2′-fluoro-2′-C-methyl-D-ribonofuranosyl nucleoside analogues.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for preparing a compound of Formula (I) and Formula (II), which are a key intermediates in the synthesis of certain nucleoside analogues, including 2′-branched nucleoside analogs.

wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle; and X₁ is halogen.

In one embodiment, the present invention further provides a process for preparing compounds of Formula (A) and Formula (B), which are key intermediates in the synthesis of certain nucleoside analogues, including 2′-branched nucleoside analogs.

wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle and X₁ is halogen.

DEFINITIONS

Whenever a range is referred to within the specification, such as C₁₋₁₀ alkyl, the range independently refers to each element. For example, C₁₋₁₀ alkyl refers independently to C₁-alkyl, C₂-alkyl, C₃-alkyl, C₄-alkyl, C₅-alkyl, C₆-alkyl, C₇-alkyl, C₈-alkyl, C₉-alkyl and C₁₀-alkyl.

The term alkyl, as used herein, unless otherwise specified, includes a saturated straight, branched, or cyclic hydrocarbon, including but not limited to those of C₁ to C₁₀, and preferably C₁-C₄, including methyl, ethyl, propyl, isopropyl, cyclopropyl, methylcyclopropyl, butyl, isobutyl, t-butyl, sec-butyl, cyclobutyl, and (cyclopropyl)methyl. Cycloalkyl groups include groups with 3 to 10 carbons. The alkyl group specifically includes fluorinated alkyls such as CF₃ and other halogenated alkyls such as CH₂CF₂, CF₂CF₃, the chloro analogs, and the like.

The term alkenyl, as used herein, unless otherwise specified, includes a C₂ to C₁₀ hydrocarbon with at least one double bond, including but not limited to vinyl.

The term alkynyl, as used herein, unless otherwise specified, includes a C₂ to C₁₀ hydrocarbon with at least one triple bond, including but not limited to acetylene.

The term alkenyl, as used herein, unless otherwise specified, includes a C₃ to C₁₀ hydrocarbon with at least two double bonds that share a central carbon atom.

The alkyl, alkenyl and alkynyl groups can be optionally substituted with one or more moieties selected from the group consisting of aryl, heteroaryl, heterocyclic, carbocycle, alkoxy, heterocyclooxy, heterocycloalkoxy, aryloxy, arylalkoxy, heteroaryloxy; heteroarylalkoxy, carbohydrate, amino acid, amino acid esters, amino acid amides, alditol, halo, haloalkyl, hydroxyl, carboxyl, acyl, acyloxy, amino, amido, alkylamino, dialkylamino, arylamino, nitro, cyano, thiol, imide, sulfonic acid, sulfate, sulfonyl, sulfanyl, sulfinyl, sulfamoyl, carboxylic ester, carboxylic acid, amide, phosphonyl, phosphinyl, phosphoryl, thioester, thioether, oxime, hydrazine, carbamate, phosphonic acid, phosphate, phosphonate, phosphinate, sulfonamido, carboxamido, hydroxamic acid, sulfonylimide, substituted or unsubstituted urea connected through nitrogen including but not limited to NHCONH₂ and NHCONHR; or any other desired functional group that does not inhibit the pharmacological activity of this compound, either unprotected, or protected as necessary, as known to those skilled in the art, for example, as taught in Greene, et al., Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, hereby incorporated by reference.

The term aryl, as used herein, and unless otherwise specified, includes phenyl, biphenyl, or naphthyl, and preferably phenyl. The term aryl includes heteroaryl groups. The aryl group can be optionally substituted with one or more of the moieties selected from the group consisting of alkyl, heteroaryl, heterocyclic, carbocycle, alkoxy, aryloxy, aryloxy, arylalkoxy, heteroaryloxy; heteroarylalkoxy, carbohydrate, amino acid, amino acid esters, amino acid amides, alditol, halo, haloalkyl, hydroxyl, carboxyl, acyl, acyloxy, amino, amido, alkylamino, dialkylamino, arylamino, nitro, cyano, thiol, imide, sulfonic acid, sulfate, sulfonyl, sulfanyl, sulfinyl, sulfamoyl, carboxylic ester, carboxylic acid, amide, phosphonyl, phosphinyl, phosphoryl, thioester, thioether, oxime, hydrazine, carbamate, phosphonic acid, phosphate, phosphonate, phosphinate, sulfonamido, carboxamido, hydroxamic acid, sulfonylimide or any other desired functional group that does not inhibit the pharmacological activity of this compound, either unprotected, or protected as necessary, as known to those skilled in the art, for example, as taught in Greene, et al., “Protective Groups in Organic Synthesis,” John Wiley and Sons, Second Edition, 1991. Alternatively, adjacent groups on the aryl ring may combine to form a 5 to 7 membered carbocyclic, aryl, heteroaryl or heterocylic ring. In another embodiment, the aryl ring is substituted with an optionally substituted cycloalkyl (such as cyclopentyl or cyclohexyl), or an alkylene dioxy moiety (for example methylenedioxy).

The term heterocyclic or heterocycle includes nonaromatic cyclic groups that may be partially (contains at least one double bond) or fully saturated and wherein there is at least one heteroatom, such as oxygen, sulfur, nitrogen, or phosphorus in the ring. The term heteroaryl or heteroaromatic, as used herein, includes aromatic groups that include at least one sulfur, oxygen, nitrogen or phosphorus in the aromatic ring. Nonlimiting examples of heterocylics and heteroaromatics are pyrrolidinyl, tetrahydrofuryl, piperazinyl, piperidinyl, morpholino, thiomorpholino, tetrahydropyranyl, imidazolyl, pyrrolinyl, pyrazolinyl, indolinyl, dioxolanyl, or 1,4-dioxanyl. aziridinyl, furyl, furanyl, pyridyl, pyrimidinyl, benzoxazolyl, 1,2,4-oxadiazolyl, 1,3,4-oxadiazolyl, 1,3,4-thiadiazole, indazolyl, 1,3,5-triazinyl, thienyl, isothiazolyl, imidazolyl, tetrazolyl, pyrazinyl, benzofuranyl, quinolyl, isoquinolyl, benzothienyl, isobenzofuryl, pyrazolyl, indolyl, isoindolyl, benzimidazolyl, purinyl, carbazolyl, oxazolyl, thiazolyl, benzothiazolyl, isothiazolyl, 1,2,4-thiadiazolyl, isooxazolyl, pyrrolyl, quinazolinyl, cinnolinyl, phthalazinyl, xanthinyl, hypoxanthinyl, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, 1,2,3-oxadiazole, thiazine, pyridazine, or pteridinyl, wherein said heteroaryl or heterocyclic group can be optionally substituted with one or more substituent selected from the same substituents as set out above for aryl groups. Functional oxygen and nitrogen groups on the heteroaryl group can be protected as necessary or desired. Suitable protecting groups can include trimethylsilyl, dimethylhexylsilyl, t-butyldimethylsilyl, and t-butyldiphenylsilyl, trityl or substituted trityl, alkyl groups, acyl groups such as acetyl and propionyl, methanesulfonyl, and p-toluenesulfonyl.

The term aralkyl, as used herein, and unless otherwise specified, refers to an aryl group as defined above linked to the molecule through an alkyl group as defined above. The aryl and alkyl portions can be optionally substituted as described above.

The term halo or halogen, as used herein, includes chloro, bromo, iodo and fluoro.

The term acyl, as used herein, refers to a group of the Formula C(O)R′, wherein R′ is an alkyl, aryl, alkaryl or aralkyl group, or substituted alkyl, aryl, aralkyl or alkaryl, wherein these groups are as defined above.

The term purine or pyrimidine base includes, but is not limited to, adenine, N⁶-alkylpurines, N⁶-acylpurines (wherein acyl is C(O)(alkyl, aryl, alkylaryl, or arylalkyl), N⁶-benzylpurine, N⁶-halopurine, N⁶-vinylpurine, N⁶-acetylenic purine, N⁶-acyl purine, N⁶-hydroxyalkyl purine, N⁶-thioalkyl purine, N²-alkylpurines, N²-alkyl-6-thiopurines, thymine, cytosine, 5-fluorocytosine, 5-methylcytosine, 6-azapyrimidine, including 6-azacytosine, 2- and/or 4-mercaptopyridine, uracil, 5-halouracil, including 5-fluorouracil, C⁵-alkylpyrimidines, C⁵-benzylpyrimidines, C⁵-halopyrimidines, C⁵-vinylpyrimidine, C⁵-acetylenic pyrimidine, C⁵-acyl pyrimidine, C⁵-hydroxyalkyl purine, C⁵-amidopyrimidine, C⁵-cyanopyrimidine, C⁵-nitropyrimidine, C⁵-amino-pyrimidine, N²-alkylpurines, N²-alkyl-6-thiopurines, 5-azacytidinyl, 5-azauracilyl, triazolopyridinyl, imidazolopyridinyl, pyrrolopyrimidinyl, and pyrazolopyrimidinyl. Purine bases include, but are not limited to, guanine, adenine, hypoxanthine, 2,6-diaminopurine, and 6-chloropurine. Functional oxygen and nitrogen groups on the base can be protected as necessary or desired. Suitable protecting groups are well known to those skilled in the art, and include trimethylsilyl, dimethylhexylsilyl, t-butyldimethylsilyl, and t-butyldiphenylsilyl, trityl, alkyl groups, and acyl groups such as acetyl and propionyl, methanesulfonyl, and p-toluenesulfonyl.

The term “protected” as used herein and unless otherwise defined refers to a group that is added to an oxygen, nitrogen, sulfur or phosphorus atom to prevent its further reaction or for other purposes. A wide variety of oxygen and nitrogen protecting groups are known to those skilled in the art of organic synthesis.

In one embodiment, the invention provides a process of producing a 2-fluoro-2-C-substituted-1,5-lactone compound of Formula (II) which includes:

a) providing a compound of structure (i) or (ii)

where OR is a suitable leaving group and X halogen; and

Wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, aralkyl including benzyl, or heterocycle; and

b) contacting the compound with a fluorinating agent under conditions that allow replacement of the leaving group with a fluorine atom.

Leaving groups OR include but are not limited to arylsulfonate, including p-toluenesulfonate (tosylate), alkylsulfonate including methanesulfonate (mesylate), trifluoromethanesulfonate (triflate), allylsulfonate, 4-nitrobenzenesulfonate (nosylate), 4-bromobenzenesulfonate (brosylate), acetate, trifluoroacetate, arylsulfate, or alkylsulfate.

In preferred embodiments, step (b) is carried out under anhydrous conditions. Anhydrous as used herein refers to the substantial absence of water, which is achieved, e.g., by conducting the reaction under an inert gas and using substantially dry reagents, for example with less than 1% or less than 0.1% water.

In one particular embodiment, the fluorinating agent is tris(dimethylamino)sulfonium difluorotrimethyl silicate (TASF).

Other, less preferred nucleophilic fluorinating agents include but are not limited to HF, HF-amine complexes, including HF-pyridine, sulfur tetrafluoride, KF, KF/crown ether, CaF, LiF, NaF, silver(I) fluoride, CsF, antimony (III) fluoride, antimony (V) fluoride, n-Bu₄NF, cyanuric fluoride, tetrabutylammonium difluorotriphenylstannate, (diethylamino)sulfur trifluoride (DAST), morpholinosulfur trifluoride (Morpho-DAST), N,N-diethyl-1,1,2,3,3,3-hexafluororopropylamine, N,N-diethyl-1,2,3,3,3-pentafluororopropenamine, N,N-diethyl(2-chloro-1,1,2-trifluoroethyl)-amine and tetrabutylammonium difluorotriphenyl stannate. Chlorinating agents include but are not limited to HCl, chloride salts, thionyl chloride, PCl₃, and PCl₅. Brominating agents include but are not limited to HBr, bromide salts, thionyl bromide, PBr₃, and PBr₅. Iodinating agents include but are not limited to HI and iodide salts. TASF is a preferred fluorinating agent because of the optimized yield of the desired product.

Substitution of the leaving groups OR or X with halogen may be achieved with any nucleophilic halogenating agents known to those skilled in the art, however, TASF is a preferred halogenating agent because of the optimized yield of the desired product.

In certain embodiments, the process produces a compound of Formula (II)

wherein R¹C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, aralkyl including benzyl, or heterocycle, in at least 40% or more yield. In certain subembodiments, the compound of Formula (II) is produced in at least 50%, at least 55%, at least 60%, at least 70% or at least 80% or more yield.

In one subembodiment, R¹ in the compound of Formula (II) is methyl. In another subembodiment, R¹ is ethyl. In another subembodiment, R¹ is vinyl. In yet another subembodiment, R¹ is —C≡CR², wherein R² is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, CF₃, cyano, aryl, benzyl, or heterocycle. In another subembodiment, R¹ is —C≡CH.

In one embodiment, the process further includes converting a compound of Formula (II) to a 1,4-lactone compound. In one embodiment, this conversion includes contacting the product from step (b) with a suitable acid. In one embodiment, the acid is an organic acid. Suitable acids include but are not limited to trifluoroacetic acid, trichloroacetic acid, acetic acid, methylsulfonic acid, p-toluenesulfonic acid, and trifluoromethylsulfonic acid. In one subembodiment, the acid is trifluoroacetic acid. In another embodiment, the acid is methanesulfonic acid. In yet another embodiment, the acid is trichloroacetic acid. In one embodiment, the solvent is 1,4-dioxane. The 1,4-lactone product can be a 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone a compound of Formula (B):

wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, aralkyl including benzyl, or heterocycle.

In certain embodiments, these compounds can be further modified by reduction of the lactone, derivitization of the resulting hydroxyl and addition of a base (including purine or pyrimidine bases) to provide a 2′-fluoro-2′-branched nucleoside including 2′-deoxy-2′-fluoro-2′-C-methyl-D-ribonofuranosyl nucleoside analogues.

In one embodiment, the process of Scheme I is provided:

wherein OR is arylsulfonate, including p-toluenesulfonate (tosylate), alkylsulfonate including methanesulfonate (mesylate), trifluoromethanesulfonate (triflate), allylsulfonate, 4-nitrobenzenesulfonate (nosylate), 4-bromobenzenesulfonate (brosylate), acetate, trifluoroacetate, arylsulfate, or alkylsulfate; and R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl, including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle.

In a subembodiment, OR is triflate and R¹ is lower alkyl. In another subembodiment, R¹ is methyl. In still another subembodiment, OR is mesylate. In another subembodiment, OR is triflate and R¹ is acetylene. In yet another subembodiment OR is triflate and R¹ is vinyl.

In another embodiment, the process of Scheme II is provided:

wherein X is chloro, bromo, iodo or fluoro. The stereochemistry at the 2-position of the lactone is inverted as the result of the displacement of the halogen group by fluoride; and

R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl, including acetylene, alkenyl, CF₃, cyano, aryl, benzyl, or heterocycle.

In a subembodiment, X is chloro and R¹ is methyl. In another subembodiment, X is bromo and R¹ is methyl. In another subembodiment, X is chloro or bromo and R¹ is acetylene. In yet another subembodiment, X is chloro or bromo and R¹ is vinyl.

In one embodiment, the process of Scheme IIIA is provided:

wherein OR is arylsulfonate, including p-toluenesulfonate (tosylate), alkylsulfonate including methanesulfonate (mesylate), trifluoromethanesulfonate (triflate), allylsulfonate, 4-nitrobenzenesulfonate (nosylate), 4-bromobenzenesulfonate (brosylate), acetate, trifluoroacetate, arylsulfate, or alkylsulfate.

In another embodiment, the process of Scheme IIIB is provided:

wherein X is halogen.

In one specific embodiment, the process of Scheme IV is provided:

wherein OTf is triflate.

In any of the above schemes, in one embodiment steps i and ii and are carried out under anhydrous conditions. In certain embodiments, the reagents are all anhydrous, including all starting materials and solvents. The introduction of the fluoro atom to the 1,5-lactone is achieved stereospecifically with inversion at the carbon. The stereochemistry of the desired product is controlled by the stereochemistry of the starting lactone.

The use of tris(dimethylamino)sulfonium difluorotrimethyl silicate (TASF) as the fluorinating agent in specific amounts of equivalents and in a specific method of addition to the reaction mixture, i.e., lack of exposure to atmospheric conditions due to the hygroscopic nature of TASF, results in the formation of 2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone as the major product, rather than the elimination byproduct 4. The 2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone then can be converted to 2-deoxy-2-halo-2-C-methyl-D-ribono-1,4-lactone as a major product by treatment with acid. 2-deoxy-2-halo-2-C-methyl-D-ribono-1,5-lactone is formed as a minor byproduct.

Step i:

Often hydroxy groups on a reactant molecule can be prevented from participating in a reaction by using protecting groups known to those of skill in the art and as taught, for example, by Greene and Wuts, Protective Groups in Organic Synthesis (1999), Third Ed., John Wiley & Sons, Inc., New York, N.Y. Common hydroxy-protecting groups include ethers, esters, particularly benzoyl groups.

In one embodiment, the 2-OH group of a 3,4-O-isopropylidene-2-C-methyl-D-arabinono-1,5-lactone is converted to an leaving group to facilitate substitution with a halogen atom. In one embodiment, the preparation of the 2-OR leaving group is formed under anhydrous conditions. The temperature can be reduced to below 0° C., in certain embodiments to below −10° C., below −20° C., below −30° C. or below −40° C.

In one embodiment, the leaving group OR is arylsulfonate, including p-toluenesulfonate (tosylate), alkylsulfonate including methanesulfonate (mesylate), trifluoromethanesulfonate (triflate), allylsulfonate, 4-nitrobenzenesulfonate (nosylate), 4-bromobenzenesulfonate (brosylate), acetate, trifluoroacetate, arylsulfate, or alkylsulfate. Starting compounds for the synthesis of this invention can be obtained commercially. For example, 3,4-O-isopropylidene-2-C-methyl-D-arabinono-1,5-lactone, is obtainable commercially from a company such as Prime Organics Ltd. Other compounds that can be used as starting materials for this process include protected 2-C-methyl-D-arabinono-1,4-lactone.

Step ii:

In certain embodiments, a halogen substituted compound is used as starting material and is commercially obtained. In these instances, a separate step to include a leaving group (i.e. step i) is not required.

The 2-C-methyl-1,5-lactone product of step i or a commercially available 2-C-methyl-2-halogenated-1,5-lactone is reacted with a fluorinating agent, under conditions that allow substitution of the leaving group. This reaction is typically carried out under anhydrous conditions. In one embodiment, the reaction contains less than 1%, less than 0.1%, or less than 0.01% water.

In one embodiment, the fluorinating agent is tris(dimethylamino)sulfonium difluorotrimethyl silicate (TASF). Other, less preferred fluorinating agents include but are not limited to HF, HF-amine complexes, including HF-pyridine, sulfur tetrafluoride, KF, KF/crown ether, CaF, LiF, NaF, silver(I) fluoride, CsF, antimony (III) fluoride, antimony (V) fluoride, n-Bu₄NF, cyanuric fluoride, tetrabutylammonium difluorotriphenylstannate, (diethylamino)sulfur trifluoride (DAST), morpholinosulfur trifluoride (Morpho-DAST), N,N-diethyl-1,1,2,3,3,3-hexafluororopropylamine, N,N-diethyl-1,2,3,3,3-pentafluororopropenamine, N,N-diethyl(2-chloro-1,1,2-trifluoroethyl)-amine and tetrabutylammonium difluorotriphenyl stannate.

Suitable solvents include dichloromethane, toluene, tetrahydrofuran, 1,4-dioxane, ethyl ether, acetonitrile, tert-butylmethyl ether (TBME), 2-methyl THF, dichloroethane, chloroform, isopropyl ether, xylenes, dimethoxy ethane, diethoxy methane.

This reaction is typically carried out at about equal to or less than 0° C., typically at less than −2° C., about −5° C., less than −5° C., about −10° C., or less. In one embodiment, the halogenating agent is added after conversion of an arabino-1,5-lactone to a ribono-1,5-lactone. In one embodiment, the temperature is increased to about 0° C. after contact with the halogenating agent. The substitution reaction is stereospecific, occurring with inversion at the carbon.

Step iii:

A 2-halo-2-C-substituted-D-ribono-1,5-lactone can be converted to 2-deoxy-2-halo-2-C-substituted-D-ribono-1,4-lactone by contacting the compound with an acid. The reaction produces a mixture of 1,4-lactone as the major product, with a 2-halo-3,4-O-isopropylidene-2-C-substituted-D-ribono-1,5-lactone as the minor product. This reaction can be conducted at room temperature.

In one embodiment, the acid is an organic acid. Suitable acids include but are not limited to trifluoroacetic acid, trichloroacetic acid, acetic acid, methylsulfonic acid, p-toluenesulfonic acid, and trifluoromethylsulfonic acid. In one embodiment, the acid is an organic acid. In one subembodiment, the acid is trifluoroacetic acid.

In certain embodiments, the deprotection is carried out for longer time periods and additional acid is added to maximize the formation of product. For example, in one embodiment, to provide the 1,4-lactone intermediate in greatest yields, deprotection reaction time can be increased from what was previously used. In addition, additional TFA (trifluoroacetic acid)/H₂O/dioxane can be added, and the acetone byproduct can be removed by vacuum distillation to drive the reaction over its equilibrium point.

Step iv:

The 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone can be used in a continuous synthesis to form a fluoro-containing nucleoside analogue. Hydroxy groups on a reactant molecule can be blocked from participating in a reaction by using protecting groups known to those of skill in the art and as taught, for example, by Greene and Wuts, Protective Groups in Organic Synthesis (1999), Third Ed., John Wiley & Sons, Inc., New York, N.Y. Common hydroxy-protecting groups include ethers, esters, and particularly benzoyl groups. Benzoyl chloride can easily and effectively be added to a ribonolactone intermediate in the presence of a base, to provide a 3,5-di-benzoyl protected ribonolactone.

In one embodiment, the steps for protection include: dissolving 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone in pyridine and cooling the resulting solution to about 0° C. under an inert atmosphere, including an argon or nitrogen atmosphere, adding benzoyl chloride dropwise over a short time, and then bringing the resulting solution to room temperature and stirring for about 3-5 hours. Additional benzoyl chloride then is added and the solution stirred for about 15-20 hours, after which a further amount of benzoyl chloride is added. The solution is stirred about another 2-4 hours, a final portion of benzoyl chloride added, and the solution was aged for about another hour. The reaction is then quenched by the addition of water, and the solids formed during the reaction are dissolved. After stirring the solution for about 5 minutes, crystals precipitate from the solution. The crystals are filtered and washed with water. The resulting solid is dissolved in dichloromethane and washed with an HCl solution. The organic layer produced from the dichloromethane/HCl treatment is dried, filtered and concentrated in vacuo to afford an off-white residue. The residue is purified by flash chromatography to provide 3,5-dibenzoyl-2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone.

In an alternate embodiment, a process is provided for preparing a compound of Formula (I), which is a key intermediate in the synthesis of certain nucleoside analogues, including 2′-branched nucleoside analogs.

wherein R¹ is C₁₋₁₀ alkyl, C₁₋₄ lower alkyl, C₃₋₈ cycloalkyl, alkenyl including vinyl, alkynyl including acetylene, alkenyl, CF₃, cyano, aryl, aralkyl including benzyl, or heterocycle; and X₁ is halogen.

In one embodiment, the present invention further provides a process for preparing a compound of Formula (A), which is a key intermediate in the synthesis of certain nucleoside analogues, including 2′-branched nucleoside analogs.

wherein R¹ and X₁ are as defined above.

Identity of all product compounds described below was confirmed by extensive NMR, MS, IR, optical rotation α-_(D), melting point, CHN elemental analysis, and/or crystal structure determinations.

EXAMPLES Example 1 3,4-O-Isopropylidene-2-C-methyl-2-O-trifluoromethanesulfonyl-D-arabinono-1,5-lactone

3,4-O-Isopropylidene-2-C-methyl-D-arabinono-1,5-lactone (7.09 g, 0.035 mol) was dissolved in anhydrous dichloromethane (110 ml). Anhydrous pyridine was added (25.5 ml, 0.315 mol) to the solution, which was then cooled to −40° C. under an argon atmosphere. Trifluoromethanesulfonic anhydride (28.4 ml, 0.168 mol) or triflate was added dropwise over 0.5 h. The mixture was allowed to warm slowly to −10° C. over 3.5 h, after which time the color became deep red and t.l.c. (ethyl acetate/heptane, 1:2) indicated conversion of starting material (R_(f) 0.33) to a major product (R_(f) 0.55). The reaction mixture was diluted with dichloromethane (500 ml) and washed with 1M HCl_(aq) (280 ml). The aqueous layer was extracted with dichloromethane (200 ml×2) and the combined organic layers were washed with brine (400 ml), dried (magnesium sulfate), filtered and concentrated in vacuo. The crude red/brown triflate (9.5 g) was purified by flash column chromatography (loaded from dichloromethane, eluted with heptane then ethyl acetate/heptane, 1:5) to give 3,4-O-isopropylidene-2-C-methyl-2-O-trifluoromethanesulfonyl-D-arabinono-1,5-lactone (5.51 g, 47%) as a white solid.

Data for C₁₀H₁₃F₃O₇S 334.27 gmol⁻¹; R_(f)=0.33, ethyl acetate/heptane, 1:5; ¹H NMR δ_(H) (400 MHz, CDCl₃): 1.37, 1.44 (6H, 2×s, C(CH₃)₂), 1.98 (3H, s, CH₃), 4.55 (1H, a-d, J_(5,5′) 13.0, H-5), 4.62 (1H, a-d, J 7.2, H-4), 4.70 (1H, a-d, J 9.2, H-3), 4.76 (1H, dd, J_(5,5′) 13.0, J_(5,4) 1.7, H-5′).

Example 2 2-Deoxy-2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone

3,4-O-isopropylidene-2-C-methyl-2-O-trifluoromethanesulfonyl-D-arabinono-1,5-lactone (5.48 g, 0.0164 mol) was dissolved in anhydrous dichloromethane (56 ml) giving a pale yellow solution which was cooled to −5° C. under an argon atmosphere. Tris(dimethylamino)sulfur trimethylsilyl difluoride, TASF, (3×5 g, 0.0538 mol) was added to the solution directly from the bottles due to its very hygroscopic nature. The solution was stirred at 0° C. for 5 min then allowed to warm to room temperature for 1 h. T.l.c. (ethyl acetate/heptane, 1:1) indicated complete conversion of the starting material (R_(f) 0.72) to a major, but faint product (R_(f)0.30). The reaction mixture was diluted with dichloromethane (420 ml) and washed with water (220 ml×2). The organic layer was dried (magnesium sulfate), filtered and concentrated in vacuo. The crude brown residue (4.4 g) was purified by flash column chromatography (loaded from dichloromethane, eluted with heptane then ethyl acetate/heptane, 1:2) to give 2-deoxy-2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone (1.36 g, 42%) as a white solid.

Data for C₉H₁₃FO₄ 204.20 gmol⁻¹; R_(f)=0.30, ethyl acetate/heptane, 1:1; ¹H NMR δ_(H) (400 MHz, CDCl₃): 1.34, 1.51 (6H, 2×s, C(CH₃)₂), 1.66 (3H, d, J_(H,F) 22.5, CH₃), 4.39 (1H, dd, J_(5,5′) 13.3, J_(5,4) 2.4, H-5), 4.48 (1H, dd, J_(5′,5) 13.3, J_(5′,4) 0.9, H-5′), 4.56-4.61 (2H, m, H-3, H-4); ¹³C NMR δ_(C) (100 MHz, CDCl₃): 19.46 (d, ²J_(C,F) 26.1, CH₃), 24.62, 26.31 (C(CH₃)₂), 68.93 (C-5), 71.84 (d, ³J_(C,F) 4.6, C-4), 78.18 (d, ²J_(C,F) 16.8, C-3), 89.82 (d, ¹J_(C,F) 192.5, C-2), 111.49 (C(CH₃)₂), 168.1 (d, ²J_(C,F) 25.1, C═O); ¹⁹F NMR δ_(F) (376 MHz, CDCl₃): −159.80 (1F, m, ³J_(F,H) 22.9, F). NMR assignments confirmed using COSY and HMQC experiments; Mass Spec m/z (APCI-): 203.3 ([M−H]⁻, 50%), 236.3 (100%).

Example 3 Data for 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,5-lactone

(C₆H₉FO₄) 164.13 gmol⁻¹;

R_(f)=0.45, ethyl acetate/heptane, 8:1; m.p.: 119-120° C. then 133-135° C.¹; [α]_(D) ²⁰: +115.025 (c, 0.9284 in CH₃CN); ν_(max) (KBr disc): 3414 cm⁻¹, 3276 cm⁻¹ (O—H), 1778 cm⁻¹ (C═O); ¹H NMR δ_(H) (400 MHz, CD₃CN): 1.57 (3H, d, J_(H,F) 23.7, CH₃), 2.19 (1H, br-s, OH), 4.04 (1H, dd, ³J_(H,F) 22.5, J_(3,4) 7.6, H-3), 4.12 (1H, br-s, OH), 4.54 (1H, dd, J_(5,5′) 12.4, J_(5,4) 6.3, H-5), 4.61 (1H, dd, J_(4,3) 7.6, J_(4,5) 6.3, J_(4,5′) 2.0, H-4), 4.80 (1H, dd, J_(5′,5) 12.4, J_(5′,4) 2.0, H-5′); ¹³C NMR δ_(C) (100 MHz, CD₃CN): 17.18 (d, ²J_(C,F) 24.5, CH₃), 66.56 (C-5), 72.43 (d, ²J_(C,F) 16.9, C-3), 79.61 (C-4), 92.98 (d, ¹J_(C,F) 179.5, C-2), 171.00 (d, ²J_(C,F) 21.5, C═O); ¹⁹F NMR δ_(F) (376 MHz, CD₃CN): −169.28 (1F, m, ³J_(F,H) 22.9, F). NMR assignments confirmed using COSY, HMQC, HMBC and nOe experiments; Mass Spec m/z (APCI-): 163.2 ([M−H]⁻, 30%), 143.2 (100%). ¹ Possible contamination with residual TFA.

Example 4 2-Deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone

2-Deoxy-2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone (1.29 g, 6.317 mmol) was dissolved in anhydrous 1,4-dioxane (36.7 ml) under an atmosphere of argon. A pre-mixed solution of trifluoroacetic acid (146.8 ml) in water (36.7 ml) was added to the mixture slowly at room temperature. After 48 h, t.l.c. (ethyl acetate/heptane, 3:1) indicated conversion of the starting material (R_(f) 0.57) to faint products (R_(f) 0.47, 0.43, 0.38). The solvents were removed in vacuo at 35° C. and then coevaporated with toluene (5 ml×2). The crude off-white/brown residue (1.27 g) was purified by flash column chromatography (pre-adsorbed onto silica from ethyl acetate, eluted with ethyl acetate/heptane, 2:1) to give 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone (862 mg, 83%, eluted second) as a white solid and 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,5-lactone (61 mg, 6%, eluted first) as a white solid, along with mixed fractions (115 mg, 11%).

Data for 2-deoxy-2-fluoro-2-C-dimethyl-D-ribono-1,4-lactone C₆H₉FO₄ 164.13 gmol⁻¹; R_(f)=0.45, ethyl acetate/heptane, 8:1; m.p.: 142-143° C.; [α]_(D) ²⁰: +129.323 (c, 0.9138 in CH₃CN); ν_(max) (KBr disc): 3412 cm⁻¹, 3274 cm⁻¹ (O—H), 1777 cm⁻¹ (C═O); ¹H NMR δ_(H) (400 MHz, CD₃CN): 1.55 (3H, d, J_(H,F) 23.5, CH₃), 2.22 (1H, br-s, OH), 3.17 (1H, br-s, OH), 3.69 (1H, dd, J_(5,5′) 13.3, J_(5,4) 3.7, H-5), 3.92 (1H, dd, J_(5′,5) 13.3, J_(5′,4) 1.5, H-5′), 4.05 (1H, dd, ³J_(H,F) 21.5, J_(3,4) 7.2, H-3), 4.32 (1H, m, H-4); ¹³C NMR δ_(C) (100 MHz, CD₃CN): 17.63 (d, ²J_(C,F) 25.3, CH₃), 60.19 (C-5), 71.85 (d, ²J_(C,F) 16.9, C-3), 83.94 (C-4), 94.55 (d, ¹J_(C,F) 178.7, C-2), 172.04 (d, ²J_(C,F) 21.5, C═O); ¹⁹F NMR δ_(F) (376 MHz, CD₃CN): −168.69 (1F, m, ³J_(F,H) 23.3, F). NMR assignments confirmed using COSY, HMQC, HMBC and nOe experiments; Mass Spec m/z (APCI-): 163.2 ([M−H]⁻, 40%), 143.2 (100%). Microanalysis: C₆H₉FO₄ calculated C, 43.91%, H, 5.53%, found C, 44.18%, H, 5.73%. Crystals grown from ethyl acetate/hexane.

Example 5 3,5-Di-benzoyl-2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone

2-Deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone (200 mg, 1.219 mmol) was dissolved in anhydrous pyridine (2.4 ml) and cooled to 0° C. under an atmosphere of argon. Benzoyl chloride (353 μl, 3.05 mmol) was added dropwise over 5 min. The solution was allowed to warm to room temperature and stirred for 4 h. Benzoyl chloride (142 μl, 1.219 mmol) was added dropwise and the mixture was stirred for 16 h after which time a further portion of benzoyl chloride (142 μl, 1.219 mmol) was added. Having stirred the solution for a further 3 h, a final portion of benzoyl chloride (142 μl, 1.219 mmol) was added and left for a further 1 h. T.l.c. (ethyl acetate/heptane, 2:3) indicated complete conversion of the starting material (R_(f) 0.16) to faintly stained, but UV active products (R_(f) 0.67, 0.63). The reaction was quenched with water (1 ml) and the solids dissolved. After stirring at room temperature for 5 min crystals precipitated from the solution which were filtered and washed with water (2 ml×2) and found by t.l.c. to contain two UV active components and pyridine. The dried, sticky solid (400 mg) was therefore dissolved in dichloromethane (10 ml) and subjected to a 1M HCl_(aq) (4 ml×2) wash. The organic layer was dried (magnesium sulfate), filtered and concentrated in vacuo. The crude off-white residue (340 mg) was purified by flash column chromatography (pre-adsorbed onto silica from dichloromethane, eluted with ethyl acetate/heptane, 1:4) to give 3,5-di-benzoyl-2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone (272 mg, 60%) as fine, white needles.

Data for 3,5-di-benzoyl-2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone C₂₀H₁₇FO₆ 372.34 gmol⁻¹; R_(f)=0.62, ethyl acetate/heptane, 2:3; m.p.: 123-125° C.; [α]_(D) ²⁰: +102.943 (c, 0.8728 in CH₃CN); ν_(max) (thin film): 1793 cm⁻¹ (C═O, α-fluoro-γ-lactone), 1733 cm⁻¹, 1717 cm⁻¹ (C═O, Bz); ¹H NMR δ_(H) (400 MHz, CD₃CN): 1.74 (3H, d, J_(H,F) 24.1, CH₃), 4.62 (1H, dd, J_(5,5′) 12.7, J_(5,4) 5.6, H-5), 4.75 (1H, dd, J_(5′,5) 12.7, J_(5′,4) 3.5, H-5′), 5.07 (1H, ddd, J_(4,3) 7.2, J_(4,5) 5.6, J_(4,5′) 3.5, H-4), 5.62 (1H, dd, ³J_(H,F) 17.7, J_(3,4) 7.2, H-3); 7.48, 7.55 (4H, 2×t, 4×H_(meta)), 7.64, 7.70 (2H, 2×t, 2×H_(para)), 8.01, 8.09 (4H, 2×t, 4×H_(ortho)); ¹³C NMR δ_(C) (100 MHz, CD₃CN): 18.88 (d, ²J_(C,F) 24.5, CH₃), 63.51 (C-5), 73.07 (d, ²J_(C,F) 14.6, C-3), 78.84 (C-4), 92.61 (d, ¹J_(C,F) 185.6, C-2), 129.51, 130.53 (2×C_(ipso)), 129.64, 129.78 (2×C_(meta)), 130.43, 130.76 (2×C_(ortho)), 134.46, 135.00 (2×C_(para)), 166.22, 166.63 (2×CO₂Bz), 170.64 (d, ²J_(C,F) 21.0, C═O); ¹⁹F NMR δ_(F) (376 MHz, CD₃CN): −164.77 (1F, m, ³J_(F,H) 24.4, F). NMR assignments confirmed using COSY, HMQC, HMBC and nOe experiments; Mass Spec m/z (ESI-): 373.1 ([M+H]⁺, 50%), 390.2 ([M+NH₄]⁺, 100%); HPLC (272 nm) R_(t)=6.17 (98%); Microanalysis: C₂₀H₁₇FO₆ calculated C, 64.51%, H, 4.60%, found C, 64.56%, H, 4.66%.

The foregoing merely is illustrative of the invention and is not intended to limit the invention to the disclosed processes and reaction conditions. Variations that are obvious to one of ordinary skill in the art are intended to be included within the spirit and scope of the invention as defined in the appended claims 

1. A process of preparing a compound of Formula II

wherein R¹ is straight chained, or branched alkyl, cycloalkyl, alkenyl, alkynyl, alkenyl, CF₃, cyano, aryl, aralkyl, or heterocycle comprising: a) providing a compound of structure (i) or (ii)

where OR is a leaving group and X is halogen; b) contacting the compound with a fluorinating agent under anhydrous conditions; and c) removing the hydroxyl protecting ketal group from the product of step b).
 2. The process of claim 1 wherein the fluorinating agent is tris(dimethylamino)sulfonium difluorotrimethyl silicate (TASF).
 3. The process of claim 1 wherein a compound of Formula (II) is produced in at least 40% or more yields.
 4. The process of claim 1 wherein R′ is methyl.
 5. The process of claim 1 wherein R¹ is ethyl.
 6. The process of claim 1 wherein R¹ is vinyl.
 7. The process of claim 1 wherein R¹ is —C≡CH.
 8. The process of claim 1 wherein OR is selected from the group consisting of triflate, mesylate, and tosylate.
 9. The process of claim 1 wherein X in Formula (II) is bromine, chlorine or iodine.
 10. The process of claim 8 wherein OR is triflate.
 11. The process of claim 1 wherein the reaction contains less than 1% water.
 12. The process of claim 1 wherein the reaction contains less than 0.1% water.
 13. The process of claim 1 wherein the reaction contains less than 0.01% water.
 14. The process of claim 1 further comprising converting a compound of Formula (II) to a 1,4-lactone compound by contacting the product from step (b) with an organic acid in an organic solvent.
 15. The process of claim 14 wherein the acid is trifluoroacetic acid.
 16. The process of claim 14 wherein the acid is an aryl or alkyl sulfonic acid.
 17. The process of claim 16 wherein the acid is methanesulfonic acid.
 18. The process of claim 16 wherein the acid is p-toluenesulfonic acid.
 19. The process of claim 14 wherein the solvent is 1,4-dioxane.
 20. The process of claim 14 wherein 1,4-lactone compound is a compound of Formula (B)

wherein R¹ is C₁₋₄ lower alkyl, alkenyl, alkynyl or CF₃.
 21. The process of claim 14 wherein the 1,4-lactone compound is a 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone.
 22. The process of claim 20 wherein R¹ is acetylene.
 23. The process of claim 20 wherein R¹ is vinyl.
 24. The process of claim 14 further comprising: d) reducing the 1,5-lactone product of step c) to a hemiacetal, e) derivatizing the hydroxyl from step d) to a suitable leaving group; and f) contacting the product of step e) with a purine or pyrimidine base to provide a 2′-branched nucleoside compound.
 25. The process of claim 1, wherein the compound of Formula (i) is 3,4-O-isopropylidene-2-C-methyl-2-O-trifluoromethanesulfonyl-D-arabinono-1,5-lactone.
 26. The process of claim 1 wherein a 2-deoxy-2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone is produced.
 27. The process of claim 1 wherein step (b) is carried out at a temperature below 0° C.
 28. The process of claim 1 wherein step (b) is carried out at a temperature below −5° C.
 29. A process for preparing a 2-deoxy-2-alkyl-2-halo-lactone compound comprising: a) reacting the 2-position of an isopropylidene, 2-alkyl-substituted arabinono-1,5-lactone compound with a leaving group reagent, to provide a 2-deoxy-2-alkyl-substituted-3,4-isopropylidene arabinono-1,5-lactone substituted with a leaving group at the 2-position selected from the group consisting of p-toluene sulfonate, methanesulfonate, trifluoromethanesulfonate, allylsulfonate, 4-nitrobenzenesulfonate, 4-bromobenzenesulfonate, acetate, trifluoroacetate, arylsulfate and alkylsulfonate; b) reacting the product from step a) with a fluorinating agent, a chlorinating agent, a brominating agent or an iodinating reagent to provide a 2-deoxy-2-halo-2-alkyl-substituted-3,4-isopropylidene ribono-1,5-lactone; c) deprotecting the product from step b) with an acid to provide a 2-deoxy-2-halo-2-alkyl-substituted ribono-1,4-lactone as a major product and a 2-deoxy-2-halo-2-alkyl-substituted ribono-1,5-lactone as a minor product; and d) optionally protecting one or more hydroxy groups on either one of the products from step c.
 30. The process of claim 29, step a) wherein the 2-deoxy-2-alkyl-substituted isopropylidene arabinono-1,5-lactone substituted with a leaving group at the 2-position is 3,4-O-isopropylidene-2-C-methyl-2-O-trifluoromethanesulfonyl-D-arabinono-1,5-lactone.
 31. The process of claim 29, step b) wherein the 2-deoxy-2-halo-2-alkyl-substituted-3,4-isopropylidene ribono-1,5-lactone is 2-deoxy-2-fluoro-3,4-O-isopropylidene-2-C-methyl-D-ribono-1,5-lactone.
 32. The process of claim 29, step c) wherein the 2-deoxy-2-halo-2-alkyl-substituted ribono-1,4-lactone major product is 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,4-lactone.
 33. The process of claim 29, step c) wherein the 2-deoxy-2-halo-2-alkyl-substituted ribono-1,4-lactone minor product is 2-deoxy-2-fluoro-2-C-methyl-D-ribono-1,5-lactone.
 34. The process of claim 29, step d) wherein benzoyl is the protecting group on one or more hydroxy groups on either of one of the products from claim 1, step c).
 35. The process of claim 29 wherein the fluorinating agent is TASF.
 36. The process of claim 29, step c) wherein the deprotecting reagent is trifluoroacetic acid (TFA).
 37. The process of claim 29, step a) wherein the leaving group is selected from the group consisting of triflate, mesylate.
 38. The process of claim 37 wherein the leaving group is triflate. 